1. Field of the Invention
The present invention relates to telecommunications systems and, in particular, to a method for simplifying the signal processing requirements for transmission of 2B1Q coded signals.
2. Discussion of the Prior Art
In high-speed data transmission applications over telephone twisted pair cable, the requirement for a low error rate is essential to the stable operation of the communications channel. When digital data is transmitted at a high rate over a telephone line, the main impairments are attenuation, dispersion and crosstalk noise. In addition, when signal transmission is fully duplex, the echo signal originating from the transmitter interferes with the data being received.
An example of a high speed data transmission application is the digital subscriber loop in the evolving Integrated Services Digital Network (ISDN).
The required data transmission rate over digital subscriber lines in the ISDN is 160 kbits/sec. To improve ISDN data transmission performance against the impairments mentioned above, the data to be transmitted is coded into a bandwidth-reducing format before it is transmitted.
Different modulation techniques can be employed to reduce the bandwidth of a transmitted signal. The modulation technique adopted in the United States by the American National Standards Institute (ANSI) for use over the digital subscriber line is called 2B1Q line coding. 2B1Q coding is a technique whereby a binary input sequence is transformed into a quaternary sequence by amplitude modulating pairs of bits in the binary stream of data.
According to the 2B1Q coding technique, successive pairs of binary data B=(0,1) are one-to-one mapped onto corresponding units of quaternary symbols Q=(-3, -1, +1, +3) and then transmitted as a corresponding voltage level at half the rate of the binary sequence. Therefore, an incoming binary data stream B with a bit rate of 160 kbit/sec is converted into a quaternary bit stream Q with a symbol rate of 80 kbit/sec and then transmitted. The conventional mapping rule from the binary data format to the 2B1Q data format is provided in Table 1 below.
TABLE 1 ______________________________________ Binary Quaternary B Q ______________________________________ 00 +1 01 +3 10 -1 11 -3 ______________________________________
The signal processing that is performed on a 2B1Q transmission in the data detection process is intended to remove echo interference in the received signal that is generated by the transmitter and to remove the distortion in the received signal itself. The former filtering task is referred to as echo cancellation and the latter is known as equalization.
Both echo cancellation and equalization filtering functions utilize a similar structure in that they can be performed by the same hardware configuration. One such configuration is the linear transversal filter. The following discussion of both the conventional method for performing echo cancellation/equalization and of the data coding technique of the present invention is based on the assumption that equalization is essentially the same as echo cancellation, but with a different input signal to its filter.
In the ISDN, the transmitted signal couples to the telephone twisted pair cable via a transformer, while the same transformer couples the twisted pair cable and the receiver. Therefore, an interference path exists between the transmitter and the receiver that causes every symbol that is transmitted to "echo" back into the receiver. Hence, the term "echo path" is used to identify the transfer function from the transmitter to the receiver at the same end of the digital subscriber line.
A conventional non-adaptive linear transversal filter of the type shown in FIG. 1 can be used to model the echo path. It performs the convolution between the transmitted signal, e.g. 2B1Q symbols q(n), where Q=(-3, -1, +1, +3), and the filter's individual tap gain coefficients a.sub.j, to arrive at an output y(n), where ##EQU2## and where the coefficients a.sub.j are samples of the echo pulse response of length N+1.
As shown in FIG. 1, a conventional non-adaptive linear transversal filter consists of a delay line which is tapped at intervals corresponding to the symbol width. Each tap along the delay line is connected through an amplifier to a summing device that provides the filter output y(n). The tap gains, or coefficients a.sub.j, are set to subtract the effects of interference from symbols that are adjacent in time to the desired symbol. As stated above, this transversal filter structure can be used as an echo canceller, acting as an echo predictor and cancelling the transmitter-to-receiver coupling.
In practice, due to the slow time variability of the echo path, and more importantly, due to the unknown initial values for the tap gain coefficients a.sub.j, an adaptive filter is required for echo cancellation and equalization.
Adaptive filters are frequently constructed as transversal, or tapped delay-line filters. A typical adaptive filtering application, shown in FIG. 2, involves driving a channel with an unknown impulse response with a known input signal q(n). The output of the channel at time n is given by the convolution sum y(n). In adaptively tracking the channel output, the adaptive filter produces an output x(n) which is the result of the summing of the tapped delay line outputs. Being time variable, the adaptive filter tap gain coefficients a.sub.j are iteratively updated based on a convergence algorithm which operates on the channel output y(n) and the filter output x(n) to develop an error feedback signal Ke(n) to the adaptive filter. Through a number of iterations, convergence of the channel output x(n) and the filter output y(n) is brought within acceptable limits.
In the case of both non-adaptive and adaptive filtering, the transfer function performs the convolution between the transmitted 2B1Q signals q(n) and the filter's delay line tap gains a.sub.j to arrive at the filter output y(n), where ##EQU3##
Because the symbols q(n-j) equal the transmitted 2B1Q code Q=(-3, -1, +1, +3), every multiplication operation in the convolution summation can potentially involve either a +3a.sub.j term or a -3a.sub.j term. Implementing multiplication of these terms requires splitting the product 3a.sub.j =2a.sub.j +a.sub.j. Thus, compared with 2a.sub.j multiplication for example, which is a basic left shift in binary arithmetic, implementation of 3a.sub.j multiplication requires either more clock cycles or a faster clock with an associated increase in power consumption.